This invention relates to semiconductor devices and, more particularly, to GaAs semiconductor devices with an electrode structure subject to minimum deterioration.
Planar semiconductor devices usually have a structure of a semiconductor substrate, an ohmic contact electrode formed thereon, and a second electrode formed on the first electrode or ohmic contact electrode for use as a bonding electrode or interconnection electrode. The second electrode is required to be free from the formation of intermetal compounds with the ohmic contact electrode, to be capable of being in good contact with the material to be bonded, to have low resistivity, and to have a satisfactory thermal press bonding property when it is used as a bonding electrode.
In a prior art GaAs semiconductor device, the ohmic contact electrode is made of an alloy primarily composed of gold (e.g., Au-Ge alloys and Au-Si alloys for n-type substrates and Au-Zn alloys and Au-Be alloys for p-type substrates), while the second electrode is made of aluminum or gold.
Where an n-type Ga-As substrate is used, a platinum or nickel layer is provided on the Au-Ge layer as an ohmic contact electrode in order to prevent the phenomena of ball-up in the Au-Ge layer and oxidation of Ge which would otherwise occur during a thermal treatment of wafer due to the fact that the Au-Ge layer has a low wetting property with respect to the n-type Ga-As substrate.
However, the platinum or nickel layer is formed to a very small thickness, so that gold in the adjacent Au-Ge layer will be diffusedly precipitated on the surface of this electrode layer during the thermal treatment. As mentioned earlier, the second electrode is made of either aluminum or gold. Where aluminum is used for the second electrode, it will be in direct contact with the precipitated gold, resulting in the formation of AuAl.sub.2, Au.sub.2 Al or a mixture of these compounds. To prevent the formation of these intermetal compounds, a titanium layer is provided as buffer layer between the second electrode and ohmic contact electrode. While the second electrode is used for either interconnection electrode or bonding pad electrode as mentioned, in the latter case gallium in the Ga-As substrate is caused to be diffused through the individual layers to be precipitated on the surface of the second electrode. Deterioration of the thermal press bonding property of the second electrode due to oxidation of gallium is prevented by.
Despite the measures mentioned above, the prior art Ga-As semiconductor device has the following problems. First, where gold is used for the second electrode, the material cost is expensive. In addition, it is very disadvantageous that available wire bonding equipment cannot be used. More specifically, in the case of equipment for manufacturing a semiconductor device with a silicon substrate, the employed wire bonder is set to conditions suited to the aluminum used. Therefore, temperature and load conditions have to be varied for each wire bonding when gold is used, so that the production efficiency is reduced. Of course, the installation of new equipment in the case where gold is used increases the manufacturing cost.
Where aluminum is used for the second electrode, there arises the following problem. FIG. 1 shows a Ga-As semiconductor diode which uses aluminum for the second electrode. The illustrated semiconductor device is obtained by forming an n.sup.+ -type region 2 in the surface area of a Ga-As substrate, forming an ohmic contact elecrtrode 3 on the n.sup.+ -type region, and forming a second electrode 4 on the ohmic contact electrode. The ohmic contact electrode 3 has a two-layered structure of a platinum layer and an Au-Ge layer, while the second electrode has a structure of an aluminum layer and a titanium layer, as mentioned before. In this laminated structure, however, the second electrode covers a smaller area than the first electrode. This means that the aluminum and titanium layers of the second electrode are exposed around the edges thereof during a process of fablicating an element. The corresponding edges of the two electrodes are spaced apart to an extent substantially equal to the thickness of the titanium layer in the second electrode. Nevertheless, Al-Au compounds will be formed in the neighborhood of the exposed edges of the second electrode due to outward diffusion of aluminum therein, which is caused by a thermal load in a thermal treatment in the element fabrication process. The Al-Au compounds thus formed will remain as seed after the completion of the Ga-As semiconductor device, and it gradually grows in the electrode. With the growth of the Al-Au compounds, the resistivity is increased to deteriorate the performance of the device.